The invention relates to an inertial sensor for measuring transient changes in rotational motion.
Rotational acceleration sensors or velocity sensors that determine the rotational motion of a sensor support by measuring the relative rotation of an inertial mass movable relative to the sensor support are known in numerous embodiments. These include simple centrifugal sensors, electromechanical accelerometers with mercury reaction masses and electrically powered gyroscopes. However, these known sensor systems are frequently very expensive from the mechanical and electronic standpoints and are prone to wear. In particular, they are unable to detect with sufficient accuracy those transient rotational states in which the angular velocity of the sensor support briefly reaches high peak values.
A typical application for such sensors is the acquisition of measurement data from motor vehicle accidents in which rotational accelerations on the order of 10,000.degree./sec.sup.2 and rotational velocities as high as 1,000.degree./sec are reached, and the rotational angle traversed must be determined with a high degree of accuracy for accident analysis. Such accuracy, however, is not possible with the known inertial systems, either because their measurement ranges are too low or because interfering measurement signal overshoots occur.
The goal of the present invention is to provide an inertial sensor of the type recited above which ensures an exact determination of transients, especially of extremely short changes in the rotational state of the sensor support, despite mechanically and electronically simple low-wear design.
The goal is achieved by the inertial sensor according to the invention, which operates on the basis of a simple transfer function free of nonlinearities, and consists of a few easily manufactured and assembled elements. The linear damping characteristic provided by the torque coupler according to the invention, produces effective stabilization of the measurement results relative to disturbing influences caused by torsional vibration and overshoot effect, even with brief, very high angular accelerations of the sensor support. This makes it possible to determine unambiguously from the relative angular displacement between the flywheel and the sensor housing, the change in rotation undergone by the sensor support in inertial space (in particular the size of the rotational angle traversed) and with very low computing effort, with the peripheral electronics required for signal processing and evaluation likewise being considerably simplified.
For further structural simplification, the torque coupler between the flywheel and sensor support is advantageously designed as an induction brake with a torque characteristic that is a linear function of the angular velocity. In an especially preferred embodiment the torque coupler is designed as an eddy current brake with a permanent magnet integral with the flywheel and magnetic yoke integral with the sensor support, and is provided with an annular sleeve as a conducting layer.
The coasting of the flywheel relative to the sensor support results from the changes in rotation of the sensor support, and the linear damping characteristic. It is advantageous to improve the measurement stability and accuracy by a selection of a time constant, which is determined by the inertial mass of the flywheel and the steepness of the linear torque curve, several times greater than the duration of the rotational changes of the sensor support.
The rotational angle traversed by the sensor support during the change in rotation can be determined very simply and without costly integration, from the measurement signals of the angular position detector on the basis of the linear transfer function of the inertial sensor.
For further structural simplification and cost savings, in another embodiment of the invention, the evaluation circuit constitutes a module separate from the other sensor components, and the measurement signals from the angular position detector are stored for later signal evaluation in temporary storage, resulting in substantial space-saving and an inexpensive design, especially for application to motor vehicle accident analysis as mentioned at the outset.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.